1. Field of the Invention
The present invention relates to the thermal treatment of glass sheets, particularly those that have to be heated to a precise elevated temperature requisite for further processing. Further processing might involve tempering and annealing or other heat strengthening depending upon the rate of cooling subsequent to the heating of the sheet to the elevated temperature requisite for the following treatment. Glass sheets are also heated to elevated temperatures prior to the application of a coating composition that develops into a coating on contact with the heated surface of the glass sheet. In any event, uniformity of product depends upon the uniformity of temperature of each sheet in a series of glass sheets that is processed.
While the present invention will be described in terms of a method and an apparatus for press bending and tempering glass sheets for the development of glazing closures for vehicles such as automobiles, it is understood that the present invention is also suitable for use in obtaining uniform temperature from sheet to sheet regardless of the nature of the subsequent glass sheet treatment. Where a succession of glass sheets must be treated at elevated temperature with a composition that reacts at the surface of the glass to provide a coating having desired optical or color characteristics, the uniformity of the coating from sheet to sheet is important for commercial purposes. Unless each sheet has the same characteristics as every other sheet in a series of sheets produced for a particular customer, the uniformity of appearance of the sheets in a building will be spoiled and the fabrication technique that does not provide uniformity of product from sheet to sheet leaves something to be desired.
It is well known that when glass sheets of different thicknesses are conveyed through an enclosed heating furnace at a uniform cycle of speed through the furnace from sheet to sheet, that those sheets that are relatively thin will develop a higher temperature on reaching the furnace exit whereas relatively thicker sheets will develop a lower temperature on arrival at the furnace exit. Consequently, glass sheets of different thicknesses will be shaped to different configurations, because those sheets that are relatively cold will be more difficult to bend into shape whereas the relatively thin sheets will retain a higher temperature after they are shaped and tend to lose the imparted shape until their temperature is reduced to one below which the sheet continues to deform. If the heated sheets are to be treated with a coating composition, it is evident that the different temperatures at which the composition is applied to sheets of different thicknesses will cause differences in characteristics of the resulting films.
2. Description of Patents of Interest
U.S. Pat. No. 3,744,985 to Peternel discloses a method and apparatus for controlling the furnace temperature in response to the average temperature of different regions of certain recently heated glass sheets as each heated glass sheet leaves the furnace en route to a further treatment station. The temperature control within the furnace is adjusted in the light of the recent history of glass sheet temperatures. Therefore, if glass sheets of random thicknesses are conveyed through the furnace, there is no assurance that a temperature control based on the average temperature of recently processed glass sheets would provide a suitable correction to improve the furnace temperature used to heat subsequent glass sheets.
U.S. Pat. No. 4,071,344 to Blausey discloses a glass sheet heating furnace provided with a conveyer which is automatically adjusted in response to temperature variations from a desired level detected in heated sheets as the latter leave the heating furnace en route to a further treatment station. The modification of conveyor speed is for the purpose of varying the heat exposure time of the subsequent sheets in the furnace. Any adjustment in furnace speed will affect the temperature of all the subsequent glass sheets that are conveyed through the furnace. Neither of the previous glass sheet treating furnaces are capable of applying more or less heat to individual sheets in accordance wth their respective measured temperatures. Therefore, thickness variations in any individual glass sheet passing through the furnace may tend to cause the furnace controls to surge in an oscillating manner around the desired temperature for any subsequent glass sheet.
Canadian Pat. No. 870,828 to Matsuno and Morisue relates to the operation of a multiple zone furnace for heating steel prior to a rolling operation. The furnace includes preheating, heating and soaking zones with the amount of heat in each zone being controlled. This patent recognizes problems arising from non-uniform heating of steel materials arising from various factors of non-uniformity such as varying dimensions or thicknesses of the steel material. This patented apparatus deals with steels and it is difficult to determine the temperature of the steel at different critical locations within the furnace. The temperatures are determined by indirect methods involving certain direct measurements coupled with the use of mathematical formulas. There is some suggestion of varying the velocity of a moving object in a heating zone to achieve the desired final temperature. However, the problems involved in heating steel which does not require any optical properties are much less critical than those involved in heating glass sheets to produce transparent articles having critical optical properties.
U.S. Pat. No. 3,880,635 to Jack and Richards and Canadian Pat. No. 1,031,960 to Jack, Richards, Hayward and Challis disclose thermal treatment within an enclosed furnace that supports glass sheets on its lower edge in a near vertical orientation with an edge supported on rollers inclined slightly from the vertical. The heating pattern throughout the length of the furnace is controlled in such a manner as to have the glass sheets processed through the furnace in preparation for additional treatment to develop a so-called deformation index within critical limits. These patents fail to provide means to control the temperature attained by any individual glass sheet being processed. They control the overall temperature pattern of the furnace.